linux-kernel - Re: [PATCH v11 01/14] s390/vfio-ap: No need to disable IRQ after queue reset

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Message-ID: <684234fa-03a9-71cd-14f3-ddf9b06e7e2e@linux.ibm.com>
Date:   Fri, 30 Oct 2020 17:13:38 -0400
From:   Tony Krowiak <akrowiak@...ux.ibm.com>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, freude@...ux.ibm.com, borntraeger@...ibm.com,
        cohuck@...hat.com, mjrosato@...ux.ibm.com,
        alex.williamson@...hat.com, kwankhede@...dia.com,
        fiuczy@...ux.ibm.com, frankja@...ux.ibm.com, david@...hat.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com
Subject: Re: [PATCH v11 01/14] s390/vfio-ap: No need to disable IRQ after
 queue reset

On 10/30/20 4:53 PM, Tony Krowiak wrote:
>
>
> On 10/30/20 1:54 PM, Halil Pasic wrote:
>> On Thu, 29 Oct 2020 19:29:35 -0400
>> Tony Krowiak <akrowiak@...ux.ibm.com> wrote:
>>
>>>>> @@ -1177,7 +1166,10 @@ static int vfio_ap_mdev_reset_queues(struct 
>>>>> mdev_device *mdev)
>>>>>                 */
>>>>>                if (ret)
>>>>>                    rc = ret;
>>>>> -            vfio_ap_irq_disable_apqn(AP_MKQID(apid, apqi));
>>>>> +            q = vfio_ap_get_queue(matrix_mdev,
>>>>> +                          AP_MKQID(apid, apqi));
>>>>> +            if (q)
>>>>> +                vfio_ap_free_aqic_resources(q);
>> [..]
>>
>>>> Under what circumstances do we expect !q? If we don't, then we need to
>>>> complain one way or another.
>>> In the current code (i.e., prior to introducing the subsequent hot
>>> plug patches), an APQN can not be assigned to an mdev unless it
>>> references a queue device bound to the vfio_ap device driver; however,
>>> there is nothing preventing a queue device from getting unbound
>>> while the guest is running (one of the problems mostly resolved by this
>>> series). In that case, q would be NULL.
>> But if the queue does not belong to us any more it does not make sense
>> call vfio_ap_mdev_reset_queue() on it's APQN, or?
>
> This is precisely why we prevent a queue from being taken away
> from vfio_ap (the in-use callback) when its APQN is assigned to an
> mdev in this patch series. On the other hand, this is a very good
> point.
>
>>
>> I think we should have
>>
>> if(!q)
>>     continue;
>> at the very beginning of the loop body, or we want to be sure that q is
>> not null.
>
> I agree, I'll go ahead and make this change.

After thinking about this a bit more, I don't think it makes sense to make
this change in this patch. For the current implementation, it is incumbent
upon the system administrator to ensure that a queue device is not unbound
from the vfio_ap device driver if its APQN is assigned to an mdev, so the
assumption here is that any APQN assigned to the mdev is (or was) bound to
the vfio_ap driver. If it was erroneously unbound while in use by a guest,
then both the guest and possibly the zcrypt driver will have simultaneous
access (one of the things fixed by this patch series). In that case, I think
it ought to be reset regardless of whether it is bound to vfio_ap or not.

Having said that, I think it makes sense to make the change you recommend
in patch 03/14. In that patch, the vfio_ap_queue object is retrieved 
from the
matrix_mdev. Since these queue objects are linked only when the queue
device is probed and unlinked when the the queue device is removed and
a queue device can not get bound to another driver while its APQN is 
assigned
to an mdev, it would make perfect sense to forego reset of a queue when
its APQN is assigned to an mdev.

>
>
>
>
>>
>